The following publications are believed to represent the current state of the art: U.S. Pat. Nos. 5,164,992; 5,963,670; 6,292,575; 6,819,783; 6,944,319; 6,990,217; 7,274,822 and 7,295,687; and U.S. Published Patent Application Nos. 2006/0253491 and 2007/0237355.